The basic factor of magnetic separation is the magnetic force, which acts on a particle of the substance and which is proportional to the magnetic susceptibility of the substance, the value of the magnetic induction B and the value of the gradient ∇B of the applied magnetic field. Therefore, increasing the sensitivity and selectivity of magnetic separation will require use of the highest possible values of magnetic induction and magnetic field gradient, or their united factor—the product B∇B.
It is known a magnetic separator intended for the separation of ferromagnetic materials in terms of the values of their magnetic susceptibility which makes it possible to reach a value of the product B∇B of about 4.5·105 mT2/m in a gap of a few millimeters [1]. However, this magnetic separator cannot be used for the separation of paramagnetic and diamagnetic substances and materials, because the values of the magnetic field parameters are not high enough.
It is known a magnetic system which consists of two permanent magnets with opposite magnetization in the form of a Kittel open domain structure [2]. In this system, near the edges of the faces of the joining magnets, a strong magnetic stray field appears which is caused by the non-diagonal matrix elements of the demagnetization factor tensor (see FIG. 1), and the value of the product B∇B reaches 1011 mT2/m. On the surface of magnets, in the zone of the upper edges of the joining faces (in the zone of line 0Y in FIG. 1), a strong magnetic stray field appears with the components Hy(x,z), Hz(x,z) and Hx(x,z). The component Hy(x,z) is equal to zero due to the geometry of the system, the vertical component Hz(x,z) comprises less than half the value of the induction of the magnet material, and the horizontal component Hx(x,z), which in the present case is of greatest interest, can be described by the expression:Hx(x,z)=Ms [ ln(a2+z2+2ax+x2)−2 ln(x2+z2)+ln(a2+z2−2ax+x2)], wherein:                Ms is the magnetization saturation of the magnets, and        a is the size of the magnet along the 0x axis (see FIG. 1).        
It follows from this expression that on the plane z=0, at point 0 the horizontal component of the stray field strives into infinity. As a result, in a small area −0.1a≦x≦0.1a, along the line of the joining magnets the horizontal component of the magnetic stray field makes an abrupt jump, which is noted by a dotted line in FIG. 1, the intensity of which can be several times stronger than the induction of the magnet material.
The important practical feature of the magnetic system described is the fact that the stray field Hx(x,z) possesses a high gradient, which in the area near to the point 0 can reach a values of 106-109 mT/m. In this system the value of the product B∇B reaches 1011 mT2/m. The disadvantage of this magnetic system is the impossibility of controlling the form and gradient of the created magnetic fields which causes the practical impossibility of using this system for the separation of substances and materials.
A high-gradient magnetic separator is known, which makes it possible to reach a value of the product B∇B of about 1.3·1010 mT2/m in a gap of a few micrometers [3]. The disadvantage of this separator is the necessity of introducing ferromagnetic bodies, (wires, balls, and the like) with a size of 25-60 μm into the substances being analyzed, this fact substantially limiting the possible range of properties and characteristics of the substances to be separated.
A device for continuous removal of impurities from colloidal dispersions, which contain pathogenic components, such as viruses and microbes, is known [4]. The device is supplied with at least one magnet with a central core, the poles of which are turned to one another and located in such a way that they form a channel with a magnetic field, which is perpendicular to their surfaces. In the channel there is a basket in the shape of a tray of rectangular cross-section and made from non-magnetic material, in which a filter is established from a material with high magnetic permeability, in the form of untied fibres, wires, net-like cloths or powders, which makes it possible to create a high gradient magnetic field. One side of the basket and filter communicates with a chamber for supplying the solution, and the other—with a chamber for collecting the filtered liquid. The disadvantage of this device is the necessity of introducing ferromagnetic bodies in the form of the filter, into the substances being analyzed and the impossibility of its application for the separation of non-liquid substances.
A magnetic system is known, for magnetic separation of biological substances by the method of sedimentation of particles, which can be magnetized, from the suspension [5]. This magnetic system includes a carrier plate, on which an iron plate is fixed, and a number of permanent magnets mounted on the iron plate, the polarity of each magnet being opposite of the polarity of the adjacent magnet. A magnetic field concentrator plate of iron is overlying the magnets and a cover plate is disposed above the field concentrator plate. A hole is provided in the cover plate and field concentrator plate for locating in the magnetic field, tubes with the suspension being separated. The plate of the magnetic field concentrator has a smooth external surface and a cone-shaped cross-section, such that the thickness of the plate decreases towards the holes. The disadvantage of this magnetic system is the impossibility of achieving such parameters of the magnetic field that would allow using it for the separation paramagnetic substances in terms of the magnitudes of their paramagnetic susceptibility.